---
url: >-
  https://adk.nht.io/api/@nhtio/adk/turn_runner/interfaces/ToolExecutionStartEvent.md
description: >-
  Payload emitted on the observability bus when a tool's executor begins
  executing the handler.
---

# Interface: ToolExecutionStartEvent

Defined in: [lib/types/turn\_runner.ts:138](https://github.com/NHTIO/ADK/blob/v1.20260605.0/src/lib/types/turn_runner.ts#L138)

Payload emitted on the observability bus when a tool's executor begins executing the handler.

## Remarks

Fired after arg validation passes and immediately before the handler is called.
`callId` is the cross-bus join key — equal to [TurnToolCallContent.checksum](TurnToolCallContent.md#property-checksum) and
[@nhtio/adk!ToolCall.checksum](../../forge/classes/ToolCall.md#property-checksum), **not** [@nhtio/adk!ToolCall.id](../../forge/classes/ToolCall.md#property-id). Fires for both
[@nhtio/adk!Tool](../../forge/classes/Tool.md) and [@nhtio/adk!ArtifactTool](../../forge/classes/ArtifactTool.md) invocations — the payload is identical.

## Properties

| Property                                    | Type       | Description                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        | Defined in                                                                                                        |
| ------------------------------------------- | ---------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ----------------------------------------------------------------------------------------------------------------- |
|  `args`           | `unknown`  | The validated arguments that will be passed to the handler.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        | [lib/types/turn\_runner.ts:152](https://github.com/NHTIO/ADK/blob/v1.20260605.0/src/lib/types/turn_runner.ts#L152) |
|  `callId`       | `string`   | Cross-bus join key: `sha256({ tool, args })`, identical to [TurnToolCallContent.checksum](TurnToolCallContent.md#property-checksum) and [@nhtio/adk!ToolCall.checksum](../../forge/classes/ToolCall.md#property-checksum). This is **not** [@nhtio/adk!ToolCall.id](../../forge/classes/ToolCall.md#property-id). It collides by design for identical `(tool, args)` within a turn — that is what [@nhtio/adk!DispatchContext.toolCallCount](../../types/interfaces/DispatchContext.md#toolcallcount) counts. Order or disambiguate repeated calls by `startedAt`. Empty string when not provided. | [lib/types/turn\_runner.ts:150](https://github.com/NHTIO/ADK/blob/v1.20260605.0/src/lib/types/turn_runner.ts#L150) |
|  `startedAt` | `DateTime` | When execution started.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            | [lib/types/turn\_runner.ts:154](https://github.com/NHTIO/ADK/blob/v1.20260605.0/src/lib/types/turn_runner.ts#L154) |
|  `toolName`   | `string`   | Name of the tool being executed.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   | [lib/types/turn\_runner.ts:140](https://github.com/NHTIO/ADK/blob/v1.20260605.0/src/lib/types/turn_runner.ts#L140) |
|  `turnId`       | `string`   | ID of the turn in which the tool is being executed.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                | [lib/types/turn\_runner.ts:142](https://github.com/NHTIO/ADK/blob/v1.20260605.0/src/lib/types/turn_runner.ts#L142) |